d90 and wedding

I was asked to shoot a friend's wedding in August. Being aware of the challenge it is to shoot wedding, I warned my friend that if they have lots of expectations for the pictures, they should hire a pro. But well, they insist, so I want to do my best to help them out. I have never shot a wedding, but I am not a beginner photographer, it s been a serious hobby for a while now, and I think I can probably do a better job at it than uncle X with his super camera sitting on its shelf all year long.

My lenses: 35mm 1.8 / 50mm 1.8 / tokina 11-16mm 2.8 / nikon 70-200mm 2.8 VR1/ 18-105mm kit

My body is a d90 and I am a happy strobist with 3 flashes ( with possibility to radio trigger)

I am not worried about the outdoor shots, and the moments when I may use a flash (party). But I have some concern when it comes to the ceremony or other indoor shots when I dont /cant use a flash

gear wise, and not talking about backups do you think :

1) that is fine with what i have. they will be aware of my limitations, and still ask me to do it. So live with it guys !

2) get something like an 18-50 f2.8 (possibly tamron or sigma). Could help low light without flash wise.

3) get a used d700 and 24-70 f2.8 (possibly tamron or sigma)

4) other?

I am always on a budget photography wise, so option 1 is my favorite, or maybe option 2, as it can be quite cheap if I sell my 18-105 and get a used faster tamron or sigma lens, but anyways, I am interested in some thoughts here.